Isaac, on track to hit southern US coast on Wednesday, is less powerful than hurricane that devastated city in 2005.

Isaac, though not as powerful as Katrina, has still made New Orleans residents anxious [AFP]

Residents of the low-lying southern US coast have been boarding up homes and waiting anxiously behind fortified levees as Tropical Storm Isaac rolls over the Gulf of Mexico, threatening to become a full-blown hurricane.

Forecasters predicted Isaac would power up to hurricane strength, which starts at winds of 119km per hour (kmph), later on Tuesday and be at least a Category 1 hurricane by the time it is expected to reach the swampy coast of southeast Louisiana early on Wednesday.

The focus has been on New Orleans as the massive and slow-moving storm takes dead aim at the city devastated by Hurricane Katrina seven years ago, but the impact will be felt well beyond.

The storm's winds could be felt more than 320km from the storm's centre.

Early on Tuesday, Isaac was a large and potent tropical storm packing top sustained winds of 113kmph.

The storm system was centred about 200km southeast of the mouth of the Mississippi River at and moving northwest at 19kph, according to the National Hurricane Centre in Miami.

Although Isaac's approach on the eve of the Katrina anniversary invited obvious comparisons, the storm is nowhere near as powerful as Katrina was when it struck on Aug. 29, 2005. Katrina at one point reached Category 5 status, with winds of more than 252 kph, and made landfall as a Category 3 storm.

Seen too much to stay

Simon and Crystal Naquin decided to bring their teenage sons to a shelter in New Orleans because they were afraid the camper they call home might flood, situated as it is between a navigation canal and lower Bayou Caillou.

Simon Naquin said he rode out hurricanes when he was younger, but doesn't do that anymore since seeing the damage wrought by hurricanes Andrew, Katrina and Rita.

"Now that I got kids, I've seen too much to say, 'Stay'," said Naquin, who shared a twin air mattress with his wife while their sons read and snacked on jambalaya amid a pile of blankets and next to a stash of water bottles and food.

Not far from the shelter, where the atmosphere was subdued, the lights were low and the music loud at Sue Sue's on the Bayou Sports Bar, owned by the husband-and-wife team of Sonny Diehl, 63, and Sue Diehl, 62.

The couple moved to Houma after they rode out Katrina at a New Orleans hotel.

"I think we take it more seriously down here,'' Sue Diehl said. "And everybody prepares. They get together, they help each other. It's a great community."

"Everybody helps everybody,'' Sonny said. "Not so much in New Orleans."

No trust for levees

In the city on Monday, Mayor Mitch Landrieu did not activate a mandatory evacuation. Instead, officials urged residents to hunker down and make do with the supplies they had.

Federal officials said the updated levees around New Orleans are equipped to handle storms stronger than Isaac.

But with landfall expected Wednesday, Katrina's seventh anniversary, anxiety was high, especially in the Lower 9th Ward, wiped out by Katrina after floodwalls burst and let the waters rush in.

"I don't really trust the levees,'' said Robert Washington, who planned to evacuate along with his wife and five children. "I don't want to take that chance. I saw how it looked after Katrina back here.''

He leaned over the banister of his porch railing and looked out onto empty lots where houses stood before Katrina. His neighborhood, just a few blocks away from where the floodwall protecting the Lower 9th Ward broke open, remains largely empty.

Republican convention spared

The storm left 24 dead in Haiti and the Dominican Republic, but left little damage in the Florida Keys as it blew past. It promised a soaking but little more for Tampa, where the planned Monday start of the Republican National Convention was pushed back because of the storm.

Only a fraction of an expected 5,000 demonstrators turned out in Tampa to protest Republican economic and social policies outside the convention.

Organisers blamed Isaac and a massive police presence for their weak showing. The storm had lingering effects for much of Florida, including heavy rains and isolated flooding in Miami and points north.

Governor Rick Scott said that as of Monday evening, about 80,000 customers were without power in Florida as a result of the storm.

Scott, a Republican, was returning from the convention in Tampa to Tallahassee to monitor Isaac. Bobby Jindal of Louisiana and Robert Bentley of Alabama, fellow Gulf Coast Republican governors, said they would not attend the convention at all.

Phil Bryant, Mississippi governor, delayed his travel through Wednesday, leaving open the possibility he could attend the final day of the event.

States of emergency were in effect in Louisiana, Mississippi, Alabama and Florida.